Tennis rackets are strung with the use of a stringing machine. FIG. 1 displays a standard embodiment of the prior art. A tennis racket 20 is placed in a mounting plate 60 and clamped in place. A string is threaded through grommets in the tennis racket. The string is held in place within the tennis racket by a string clamp. The free end of the string is threaded through a roller mounted within the tension head 140. The tension head 140 is incorporated with other items to comprise the tension head assembly 100. The tension head assembly 100 is mounted on the winder bar 40. The tension head assembly 100 includes a tension crank 120. Turning the tension crank 120 causes the tension head assembly 100 to move along the winder bar 40. When a string is threaded through the tension head 140, a user can turn the tension crank 120 to move the tension head 140 away from the mounting plate 60. This movement pulls on the string and creates the necessary tension in the string until it is secured in place on the racket 20.
The tension head assembly 100 is designed so that the tension in the string is set at a predetermined tension. Historically, in tennis racket stringing machines, this predetermined tension is accomplished by means of a precompressed spring 110 within the tension head assembly 100. The tension placed on the tension head 140 by the string is transferred to the precompressed spring 110 by means of a tension transfer bar 150. The tension transfer bar 150 operates as a simple lever, where the axel 145 of the tension head operates as the fulcrum and the distance between the axel 145 and the precompressed spring 110 and the axel 145 and the string are the respective arms of the lever. Traditionally, the ratio of these arms is fixed, thus the tension is changed or set by precompressing the spring 110. When the tension in the string multiplied by the distance of the string to the axel 145 matches the tension in the precompressed spring 110 multiplied by the distance of the spring 110 from the axel 145, the tension head 140 rotates along the axel 145, releasing the tension brake 130. The brake engages with the tension crank 120, preventing additional movement of the tension head assembly 100 along the winder bar 40.
The tension of the precompressed spring 110 can be manipulated and set by turning a knob 160 connected to the precompressed spring 110, causing the winding of the precompressed spring 110 to become looser or tighter. The precompressed screw is wound about a screw connected to the knob. Turning the screw changes the winding of the spring, which changes the tension. The distance of the precompression is normally very short. The screw, to which the knob 160 is mounted, and the precompressed spring 110 are set such that one unit, or partial unit, of turning changes the tension of the spring by one pound of force. Users in countries utilizing the metric system must purchase a machine set for kilograms instead of pounds since a change of tension in one pound of force is not equal to one kilogram of force. This presents a limitation. In addition, to make the distance of precompression greater, much larger spring would have to be used, which would not be practical. In addition, utilization of a precompressed spring 110 is limiting in that the spring becomes fatigued through repetitive use and constant tension. This fatigue can cause the tension in the strings attached to the racket 20 to decrease, decreasing the performance of the racket 20. Such fatigue also requires a user to take time to recalibrate the tension, lessening the effectiveness of the user and decreasing the rate of production. In addition, the fatigue of the spring requires that the spring be replaced on a frequent basis. What is needed is a means of allowing a user to set a tension on the string and the tension head 140 without utilizing a precompressed spring 110. What is needed is a tension scale large enough so a user can easily change the tension in the tension head and to adjust the tension between the English system and metric system of measurement as needed.
When a user strings a racket 20, the racket 20 is attached to the mounting plate 60. The mounting plate 60 rotates so that the user may turn the racket 20 as needed to thread a string through separate grommets. Historically, to prevent the mounting plate 60 from rotating during the threading process, a brake 70 has been installed that is utilized by a lever. When a user desires to prevent the mounting plate 60 from rotating, the user pulls a lever into a locking position, engaging the brake 70, and preventing the mounting plate 60 from rotating. Such a method is flawed. Utilizing a separate lever for locking the mounting plate 60 becomes burdensome during the stringing process. A user must lock and release the lever several times while maintaining the tension in strings which have been threaded. In addition, some users fail to engage the brake 70, leaving the mounting plate 60 movable during the stringing process. What is needed is a means of locking the mounting plate 60 in place during the stringing process without requiring a user to make additional movements.
In addition, historically, string clamps engage strings from below. In the prior art, the string clamp is positioned on a base clamp. The base clamp is positioned in the correct position on the mounting plate and then locked in place. The string clamp is then extended upwards until it engages the strings in the racket. The clamp closes in from the sides around the string until it presses the string within the clamp with sufficient pressure to prevent the string from slipping or moving. The end of the clamp is fashioned into a comb shape. This shape allows cross strings to be positioned between the fingers of the clamp during the stringing process. The prior art is limited in that the string clamp requires a user to engage two locks to utilize the clamp. The user must engage a base clamp lock and the string clamp lock. This requires additional time on the part of the user when utilizing the string clamp. Previous attempts at creating an automatic base clamp lock were either too complicated and thus unreliable, or utilized a self-locking torque feature. The self-locking torque feature utilized the string tension, which created sufficient torque on the clamp base to become self-locking This solution resulted in considerable play and required increased skill and attention of the stringer. What is needed is a simple self-locking base clamp lock which locks positionally in place when utilized.